In a small motor, a rotation detection device for detecting a rotational speed or position has been integrated into a motor rational component such as a motor shaft. For example, these rotation detection devices have been used to gain the angular positional information in brushless motors. As already known in prior art, in the brushless motor, a magnet known as a sensor magnet is installed on the rotational shaft while a magnetic induction sensor is placed near the sensor magnet. The sensor magnet forms the alternate magnetic poles along the direction of a circumference. Since the sensor magnet has been magnetized to correspond to the magnetic pole position of a permanent magnet that is installed in a motor, as the motor shaft rotates, the magnetic induction sensor detects a change in the magnetic pole position in the sensor magnet, and the detected change corresponds to the rotor rotational change.
Incidentally, the rotation detection device detects a rotational speed or position in a motor with a worm gear speed reduction device, and the detected rotational speed and position are used to control the speed reduction. See Japanese Patent Publications 2003-164113 and 2005-348525. In a motor with a worm gear speed reduction device, the motor shaft rotational drive is outputted to the output shaft via a worm or a worm wheel. For example, in an automotive power window system, a window glass is raised and lowered via a window glass lifting and lowering mechanism. In order to detect a position and a speed of the window glass, a motor with a worm gear speed reduction device is installed in the automotive power window system.
In the rotation detection device, the sensor magnet has been placed on the motor shaft inside the motor housing or on the motor shaft extending outside the motor housing. Referring to FIG. 5, a diagram illustrates a prior art technique for placing a sensor magnet on the motor shaft as disclosed in Japanese Utility Model Publication Hei 7-38967. FIG. 5A is an expanded prospective view while FIG. 5B is a partial frontal cross sectional view. A sensor magnet 20 as a detected object is held in a sensor magnet holder 10 around the outer surface of a motor shaft 30 and is juxtaposed to a commutator 40. The sensor magnet holder 10 is integrated by a resin and includes a plurality of holding portions 10C at equidistance along the circumference. Each holding portion 10C is integrally connected to a ring-shaped connecting portion 10D at a rear portion of the sensor magnet holder 10. At a frontal portion 1 of each holding portion 10C around the circumference, a frontal engagement portion 10B protrudes outwardly along the diameter. On the other hand, at a rear portion of each holding portion around the circumference, a rear engagement portion 10A protrudes outwardly along the diameter. Both of the engagement portions 10A and 10B respectively engage the corresponding end surface of the sensor magnet 20 that is press fit by the outer circumference in the holding portion 10C.
The sensor magnet 20 integrally forms a concave portion 22, and a rotation-locking convex portion 14 of the sensor magnet holder 10 engages with the concave portion 22 in order to lock the circumferential position. An outer surface of the motor shaft 30 has a position-locking groove 32 to prevent the sensor magnet 20 from moving away from the commutator 40. On the other hand, a plurality of convex portions 12 at the rear portion of the magnet holder 10 protrudes in the inward direction along the diameter. These convex portions 12 are formed to engage inside the position-locking groove 32. Furthermore, by extending a knurl 50 for fixing an armature and a commutator 40 on the outer surface of the motor shaft 30, the sensor magnet 20 and the motor shaft 30 compress the resin of the holding portion so as to be distorted along the concave and convex portions of the knurl 50. Consequently, the holding portion 10C and the motor shaft 30 are fixedly connected with each other.
The illustrated prior art rotation detection device has complex structural forms for determining the sensor magnet position and fixing the sensor magnet with respect to the magnet holder. By the same token, the prior art rotation detection device also has complex structural forms for determining the magnet holder position and fixing the magnet holder with respect to the motor shaft. Furthermore, the torque is not sufficiently large in the prior art for preventing the sensor magnet from rotating in the circumferential direction.
The illustrated prior art rotation detection device assumes its juxtaposition to the commutator. However, if the rotation detection device is placed near a worm gear, lubricating grease of the worm gear may flow over a Hall effect sensor.
The current invention solves the above described prior art problems. The motor shaft itself does not need the prior art knurl for fixing a sensor magnet or the prior art position-determining groove. One object of the current invention is to provide a sensor magnet holder that enables a sufficiently tight press fit against the motor shaft for fixing the sensor magnet with respect to the motor shaft.
Another object of the current invention is to prevent the magnet from damage by eliminating a large amount of force such as press fit as needed by prior art in placing a magnet in a magnet holder via a motor shaft. Yet another object of the current invention is to securely fix the magnet at the inserted position both in the shaft axis direction and the shaft rotational direction.
Lastly, another object of the current invention is to prevent lubricating grease of the worm gear from flowing over a Hall effect sensor.